Bone densitometer with improved point characterization

ABSTRACT

A dual energy densitometer includes a solid statement near x-ray detector which may be swept across the patient with movement of both to produce a matrix of data elements representing attenuation through the patient. Each of these data elements may be characterized as to its material type by the use of the data element values, templates representing general rules to be applied to the bone and operator input. The operator input is provided by a cursor controlled paintbrush which changes classifications of individual pixels based on a selected brush type.

This application is a divisional of Ser. No. 08/344,255 filed Nov. 23,1994 now U.S. Pat. No. 5,533,084; which is a continuation-in-part ofSer. No. 08/241,270 filed May 10, 1994 now U.S. Pat. No. 5,509,042 basedon a PCT filing of Sep. 10, 1993 PCT/US93/08515, which is acontinuation-in-part of Ser. No. 08/067,651 filed May 26, 1993 now U.S.Pat. No. 5,291,537, which is a divisional of Ser. No. 07/944,626 filedSep. 14, 1992 now U.S. Pat. No. 5,228,068; the above U.S. Pat. No.5,509,042 is also a continuation-in-part of Ser. No. 08/073,264 filedJun. 7, 1993 now U.S. Pat. No. 5,306,306, which is a continuation ofSer. No. 07/862,096 filed Apr. 2, 1992 now abandoned, which is acontinuation of Ser. No. 07/655,011 filed Feb. 13, 1991 now abandoned.

FIELD OF THE INVENTION

The present invention relates to bone densitometers and in particular,to densitometers which analyze x-ray attenuation data to distinguishbetween bone and other materials in the body to identify particularbones and measure those bones.

BACKGROUND OF THE INVENTION

Digital bone densitometry devices such as the DPX machines manufacturedby LUNAR Corporation of Madison, Wisc. or the QDR machines manufacturedby Hologic, Inc. of Waltham, Mass., are used to generate broadly basedvalues of bone character, such as bone mineral content ("BMC") or bonemineral density ("BMD"). Such information about bone character, and inparticular, about bone character in the spine is often relied on todiagnose and treat bone depletive disorders such as osteoporosis.

Traditionally, BMC and BMD measurements have been made by scanning thespine of a patient with a radiation source directed along ananterior-posterior ("AP") axis. One problem with AP scans of the spinefor BMC and BMD measurement is that the measurement of thediagnostically significant trabecular bone in each vertebra is biased bycontribution from the posterior elements of each vertebra. This isbecause bone from the posterior elements projects into theintervertebral space and overlays much of the vertebral body of an APview. Thus most of the bone of the posterior elements were invariablyincluded in the AP measurement.

To avoid these problems, manufacturers have resorted to measuring thespine from the lateral position. In the lateral position, it is argued,the region of interest can be easily limited to the vertebral bodyexcluding the posterior elements. Thus, one avoided having themeasurement biased by the posterior elements.

Nevertheless, significant problems exist with the lateral view. Becausepatient thickness is greater in the lateral view, resolution iscompromised. For the same resolution as is obtained in the AP view, inthe lateral view one must increase the flux of the x-ray beam whichleads to an increased dose. If flux was not increased, the ability todefine the margins of the vertebral body was no better and in manyinstances was worse than with the AP view. Further, most of the lateralview of the spine is obstructed by the ribs or the hip. It can beappreciated by those skilled in the art, that such an obstructionpresents a similar biasing problem as discussed above with respect tothe posterior elements in the AP view.

At best, only two vertebrae, L1 and L2, present an unobstructed lateralview and this is true only for 20 percent of the population. In thesmall percentage of the population where an unobstructed view ispossible, if the vertebrae have a pathology, such as crush fractures,the BMC or BMD measurement of those vertebrae may not be clinicallyrelevant.

SUMMARY OF THE INVENTION

The present invention improves the measurement of bone BMC or BMD in theAP direction by defining a measurement region of interest (ROI) about avertebra that avoids areas of the vertebra that are significantly biasedby the superposition of the spinal posterior elements. A digitalcomputer analyzes both the attenuation values of acquired data elementsand the location of those data elements to identify high density areaslikely caused by the posterior elements. These areas are eliminated fromthe measurement ROI.

Specifically, the vertebrae are scanned with a beam of radiationdirected in the AP direction to acquire a matrix of discrete dataelements each having a value and a defined location through thevertebra. A digital computer reviews the values of the data elements andtheir defined locations to identify individual vertebra and zones ofdata elements within the individual vertebra where the data elementsmeasure radiation substantially attenuated by the bone of both thecentrum and the spinal processes;

These zones may be located by identifying an intervertebral spaceadjacent to the vertebra and data elements within the intervertebralspace measuring radiation substantially only attenuated by spinalprocess and not by centrum to produce a reference measurement. Thereference measurement may be subtracted from a peak value of the dataelements in the vertebrae to establish a limit with those data elementswithin the vertebrae having a value greater than the limit defining thezones.

The zones are then excluded from a calculation of the physicalcharacteristic of the material of the vertebra, which is displayed.

Thus, it is one object of the invention to provide a measurement of bonedensity in the AP direction that avoids the biasing effects of thespinal processes.

This technique can be similarly applied to locating and eliminating theintervertebral spaces from the measurement of vertebral density. Herethe values of data in the region of the intervertebral space and thelocation of that data in conjunction with the known structure of thespine are used to accurately locate intervertebral spaces and toeliminate these spaces from the density measurement.

Specifically, the data elements of the vertebrae, acquired as describedabove, are sorted based on their values into bone data elementsmeasuring the physical characteristic of the vertebrae. The definedlocations of these bone elements are used to identify the spinal columnand the intervertebral spaces, and a bone integrity value for thevertebrae is determined, which excludes the intervertebral spaces.

Thus, it is another object of the invention to eliminate not only theeffects of denser posterior elements from the vertebral measurement butalso the influence of less dense regions of the intervertebral spaces.

A highly flexible and interactive method for selecting what dataelements will be included in the measurement is provided by the use of"paintbrush" cursor which allows the operator to selectively change thecharacterization of data elements by "painting" their correspondingpixels on an image of the data elements.

Specifically, a two dimensional array of pixels having valuesrepresenting the attenuation of radiation at locations through thepatient are displayed on a digital computer having a display screen anda cursor controller providing a select signal and cursor coordinates inresponse to operator commands. The digital computer receives the arrayof pixels and categorizes the pixels into at least bone pixels, softtissue pixels and neutral pixels. An image of the pixels is displayed inwhich at least one category is visually distinguishable from the others.In response to cursor coordinates from the cursor controller a cursorsymbol is moved in a path on the image. Pixels in the path have theircategorization changed when the select signal is received. A diagnosticvalue is displayed to the operator based on the bone pixels and softtissue pixels but excluding the neutral pixels, as each is affected bythe operator under cursor control.

Thus, it is another object of the invention to permit the operator tofine tune the characterization of each data element in an interactivemanner.

Other objects, advantages, and features of the present invention willbecome apparent from the following specification when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an instrument for use in the presentinvention showing a C-arm supporting at one end an x-ray sourceproducing a fan beam whose plane is aligned with a supine patient'stransverse plane and received by a linear detector at the other end ofthe C-arm, the C-arm to be scanned along an inferior/superior axis ofthe patient to produce a matrix of data elements that may be displayedon a computer;

FIG. 2 is a schematic representation of the matrix of data elementsproduced by the instrument of FIG. 1 showing a point typing of each dataelement as bone or soft tissue;

FIG. 3 is a greatly simplified representation of an image of a matrix ofdata elements acquired by the instrument of FIG. 1 showingrepresentative vertebrae of the spine as well as portions of theclavicle and ilium where each data element is represented as a pixel onthe image;

FIG. 4 is a histogram plotting the frequency of occurrence of the pixelsin the image of FIG. 3 against the value of their corresponding dataelement showing distribution of the pixels into two modes correspondingto bone pixels and soft tissue pixels;

FIG. 5 is a flow chart depicting the steps of refining the point typingof the data elements of FIG. 2 into multiple of categories based ontheir value, their spatial distribution, and operator commands;

FIG. 6 is a representation of the display of the computer of FIG. 1showing operator menus for selecting a brush type and brush size used bythe operator for changing the point typing associated with the image ofFIG. 3;

FIG. 7 is a perspective view of a vertebra showing the centrum andrearward extending spinal prosthesis;

FIG. 8 is a simplified representation of an AP bone density image suchas shown in FIG. 3 showing zones of high density caused by thesuperposition of the rearward spinal prosthesis on the centrum and theidentification of a reference area in the intervertebral space used forremoving these high density zones from the ultimate density measurement;and

FIG. 9 is a representation of a prior art display of the computersimilar to that of FIG. 6 showing definition of regions of interest thatthe intervertebral spaces and reward spinal processes as superimposedover the image of the vertebral body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Densitometry Hardware

Shown in FIG. 1 is a perspective view of a dual energy bone densitometerof the type employed in the preferred embodiment of the presentinvention.

The densitometer 10 includes a radiation source 12 and a detector 13,both of which are mounted on a rotatable C-arm 14, which extends oneither side of a supine patient 16 so as to direct and receive radiationalong a radiation axis 24 through the patient 16.

The C-arm 14 is designed to be rotated in a vertical plane as indicatedby arrows 9 as supported by a collar 15 so as to allow both an AP viewof the spine or other bones or a lateral view of the same. In thepresent invention, however, rotation of the C-arm is not required and afixed arm positioned for AP imaging may be used.

The C-arm 14 may also be moved longitudinally along the patient's bodyin a scanning direction 19 and may be positioned under the control ofservo motors under computer control as is understood in the art.

The densitometer 10 of the preferred embodiment employs a dual energyx-ray source. "Dual energy x-ray" or "polychromatic x-ray" refers toradiation at two or more bands of energy, emitted simultaneously or inrapid succession, or a single broad band energy of more than a few keVover the diagnostic imaging range. The dual energy x-ray beam is usedfor the measurements of bone character (i.e. BMC and BMD).

The radiation source 12 may provide a fan beam 23 of x-rays which iscollimated and oriented toward the vertebra such that the plane of thefan beam 23 is perpendicular to the longitudinal axis of the spine. Theorientation of the fan beam 23 perpendicular to the spine allows imagingof the spine, or other long bones generally aligned with the spine suchas the femur, with minimal distortion along the longitudinal axisresulting in the ability to measure vertebral dimensions in this axiswith greater accuracy than possible with a cone beam. For greateraccuracy in the horizontal axis, the fan beam 23 may also be oriented sothat the vertebral body or other bone is irradiated by the centerportion of the beam rather than the edges which are subject todistortion. Since the center of a fan beam 23 has little angulation, theresulting data is comparable to that obtained with a pencil beam and yeta scan can be obtained much faster.

The detector 13 is a linear array of detector elements subtending thefan beam 23 for providing simultaneous measurements along a number ofrays of the fan beam 23 associated with each such detector element.

A general-purpose digital computer 18, is programmed for use inoperating the densitometer 10 and analyzing the data obtained from thedetector and includes specialized algorithms for carrying out thecalculations required by the present invention. In addition, the presentinvention includes a data acquisition system ("DAS") for converting thesignals produced by the detector 13 to a form compatible with thecomputer 18 and a data storage device (neither of which are shown) whichmay be incorporated in the computer 18.

The computer 18 provides an electronic display 22 for outputting thedata analysis or images of the data as will be described. A "mouse" 25or other cursor control device is provided to permit the operator tocontrol a cursor (not shown in FIG. 1) on the display 22 in response tomovement of the mouse 25 over a surface by the operator. Control buttons26 on the mouse allow for additional operator input associated with theselection of menu items and modifying images on the display 22 as willbe described in more detail below.

In most general terms, during operation of the densitometer 10, theradiation source 12 emits radiation of a certain energy level or levelsalong the radiation axis 24 at defined locations along the scan. Theradiation passes through the vertebra 20 being scanned and is thenreceived by the detector 13. The analog output of the detector 13 issampled and digitized so as to produce a signal consisting of discretedata elements, each associated with a location through the patient, bythe DAS. The DAS may then transmit the digitized signal to the computer18 which stores the data in computer memory (not shown) or on a massstorage device.

When the fan beam 23 is poly-energetic, discrimination between high andlow energy attenuation of x-rays by the patient can be done by thedetector 13. Two sets of side by side detector elements may be used, oneeach selectively sensitive to high energies or to low energies. Thus,during the scan the detector 13 produces data for high and low energyimage. These two images may later be aligned and mathematically combinedto produce bone density information according to mathematical algorithmsknown in the art. Alternatively the detector 13 can be a stacked array.In this arrangement, high and low energy detector elements are stackedthe low energy detector on top of the high energy detector. A particularadvantage of the stacked array detector is that it can easilyaccommodate a multilinear array or area detector design. Such stackeddetectors are described and claimed in Barnes, U.S. Pat. Nos. 4,626,688and 5,138,167, incorporated herein by reference.

Referring now to FIGS. 1 and 2, upon completion of the scanning of thepatient 16 by the radiation source 12 and detector 13 the computer 18arranges the data elements obtained in the scan in a matrix 29 withincomputer memory. Each data element 31 of the matrix is associated with aspatial location defined by the position of the C-arm 14 when the dataelement 31 is acquired during the scan and indicated in the matrix bythe position of the data element 31 in the matrix. The spatialseparation of the defined locations of the data elements 31 isdetermined by the distance that the instrument, e.g., the radiationsource 12 and detector 13, moves between acquiring rows of data elements31 and by the separation of detector elements in the detector 13.

Each data element 31 has a relative value proportional to the amount ofradiation transmitted by the tissue at the corresponding location. Theabsorption of radiation by a tissue correlates to certain physicalproperties of that tissue. For example, bone absorbs a greater amount ofradiation than does soft tissue. The data elements 31 thus obtained arereferred to PBM for pseudo bone mineral content. The numbers are pseudovalues because they are non-calibrated and therefore dimensionless. Atthis point in the analysis, therefore only the relative differencesbetween the data elements 31 are significant, not their absolute values.While the calibration for each data element 31 could be done at thispoint, it is consumptive of computer resources, and thus is deferred andthe PBM values are used.

Processing of Densitometry Data

Referring momentarily to FIG. 9, prior art densitometers producedrelatively low resolution images 500. For this reason, it is difficultwith prior art bone densitometers for the operator to resolve thesuperior and inferior margins of the vertebral body 20 and exclude theareas of the intervertebral space 313. For similar reasons the lateralmargins of the vertebrae 20 are also difficult to resolve. Thus,manufacturers and user of such devices establish large regions ofinterest 502 which include several vertebrae 20, typically L1-L4, orL2-L4 and the lateral margins are determined using arbitrary thresholddesigned to exclude the transverse processes, but not to conform to thetrue outline of the vertebral body. Placement and shape of the ROI isapproximate. Because several vertebrae are measured, the intervertebralspace is included in the measurement.

Referring now to FIG. 3, the data elements collected during the scan maybe displayed as an image 200 where the spatial location of each dataelement 31 in the patient maps to a pixel 201 having a correspondingspatial location in the image 200; and where the value of each dataelement is interpreted as a shade of gray and/or a color of that pixel201. Data elements recording the greatest attenuation of x-ray radiationare given the lightest gray values in the image 200 so that image 200looks like a conventional x-ray radiograph with areas of bone, havingthe greatest attenuation, depicted generally as white and areas oflesser attenuation such as soft tissue and air depicted generally asblack.

The typical image 200 will show the spine 202 resolving of the variousvertebrae 20 surrounded by soft tissue 204 and by portions of otherbones of the body such as the ilium 206 and clavicle 208. The resolutionof the densitometer 10 must be such as to clearly delineate the marginsof the vertebra. In some images 200, the x-ray fan beam 23 will passoutside of the patient's body altogether and the image 200 will includeareas of air 210.

When dual energy is used, this initial image 200 may be created bycombining the high and low data values of each location to produce aneffective polyenergetic image.

Referring to FIGS. 3 and 4, the values of the data elements will begenerally spread through the range of attenuation values. Further,within those data elements measuring only bone or only soft tissue willalso vary over a range of values. Accurate computer analysis of thisdata requires that each data element and hence each pixel 201 in theimage 200 be identified as to its tissue type. This identification or"point typing" is required not only to properly calibrate the algorithmsused in employing the dual energy measurements (which need referencemeasurements of tissue types) but also to permit automated measurementof the vertebra by the computer.

Referring now also to FIG. 5, a first step of point typing is performedby examining the values of the data elements 21 of each pixel 201 in theimage 200 as indicated by process block 400. In this value-based pointtyping, each pixel 201 of the image 200 is sorted according to aplurality of attenuation ranges forming the horizontal axis of aneffective attenuation histogram 212. The vertical axis of the histogram212 indicates the number of pixels 201 of image 200 that have aparticular attenuation value. As shown in FIG. 4, typically the pixels201 will exhibit a bi-modal distribution with a first soft tissue mode214 and a second bone mode 216.

The histogram shown in FIG. 4 reflects the fact that there is a range ofpixel values and in particular pixel values that fall between the modes214 and 216. A threshold 218 having a particular attenuation value musttherefore be identified between these modes 214 and 216, for example, atthe minima of the histogram 212 between the peaks or maxima of the modes214 and 216. This threshold 218 is used to categorize each of the pixels201 of the image 200 as either bone or soft tissue based on its value.

In images 200 which include pixels associated with data elements 31 thatmeasure only air or that measure a metallic implant, additional modes211 and 213, respectively, will be present outside of the modes 214 and216 and low and high attenuations, respectively. These modes 211 and 213can be used to generate additional thresholds 217 dividing the airpixels of mode 211 from the soft tissue pixels of mode 214, andthreshold 215 dividing the artifact pixels of mode 213 from the bonepixels of mode 216.

Referring now to FIG. 2, each data element corresponding to the pixels201 of image 200 may be compared to the thresholds 217, 218, and 215 toassign them a point type 219 on type matrix 221 in addition to itsvalue. Thus, pixels 201 having attenuation values greater than thethreshold 218 (but below threshold value 215) are assigned to bonecategory "B", whereas pixels 201 having a value less than the threshold218 (but greater than threshold 217) are assigned a tissue value "T". Aborder 111 between bone elements "B" and tissue elements "T" may thus beestablished and used for further analyses of the bone, per process block406 to be described, such as the making of morphometric measurements ofa particular vertebra such as described in U.S. Pat. Nos. 5,228,068 and5,291,537 assigned to the assignee of the present application and herebyincorporated by reference.

Often this value-based point typing is alone insufficient. This isparticularly true where it may be desired to measure only a certain typeof bone, as may be the case when one is measuring bone loss inindividuals. For example, it is believed that the vertebral body of thevertebra, (the centrum) having a large proportion of trabecular bone isa more sensitive indicator of bone loss than the harder and densercortical bone found, for example, in the spinal processes. For thisreason, it may be desired to exclude, as much as possible, the denserspinal processes which arguably dilute the measurement of change in bonedensity, remaining relatively constant even as trabecular bone is lost.

Value-based point typing may also be insufficient because of measurementerrors (from noise or quantization) and variations caused by interveningtissue. For this reason, referring to process block 402 of FIGS. 3 and5, the value-based point typing is augmented by a template-based pointtyping. In template-based point typing, knowledge about the shape of atypical spinal vertebra is used to refine the point typing. Generally,template-based point typing applies rules about bone shape specific tothe bone being investigated. For example, with the spine 202, it isknown that the vertebrae 20 generally are aligned with each other alonga slowly varying spinal axis and that their width is relativelyconstant. This "template" is used to fit two boundary lines 220 to theleft and right boundaries of the spine 202 based on the value-basedpoint typing previously performed at process block 400. The boundarylines 220 are fit to the bone pixels use of well known curve fittingalgorithms which provide the best fit of a curve described by apolynomial equation of given order points so identified.

Generally, the points to which the curve is fit may be identifiedexamining the point typing of the type matrix 221 across horizontallines in the image 200 to identify the boundary pixels 201 at which thesoft tissue "T" gives way to a bone "B".

Selecting the appropriate low order curve, based on knowledge of theanatomy of a general spine, allows the spinal processes 302 projectinglaterally from the vertebra in the AP projection to be excluded from thebone measurement. In this template-based point typing, the bone outsideof the boundary lines 220 is given a neutral characterization whichmeans that it is neither classified as bone nor soft tissue but isexcluded from the calculation of bone values.

A similar template fitting may be used to accurately identify theintervertebral spaces 313. Here, vertical paths through the type matrix221 are taken and the inferior and superior borders of the vertebra 20identified by the points at which the bone characterization "B" givesway to the tissue characterization "T" and vice versa. Low order curvesfit to these points and perpendicular to the boundary lines 220accurately establish intervertebral spaces 313 which if included in abone density calculation might bias density calculations. Although theintervertebral spaces are generally not empty of pixels having a boneclassification, in part due to the projection of the rearward spinalprosthesis through the intervertebral spaces, the curve fitting processmay be adjusted to ignore these inclusions of bone to provide sharpintervertebral boundaries.

Thus, with value-based point typing 400 and template-based point typing402, a more robust characterization of each point into the categories ofbone or soft tissue is made.

Referring now to FIGS. 7 and 8, the information from the values of thedata elements 31 and their locations may be further used to identifypoints of high bone density such as represent a superposition of thespinal processes over the vertebra image. As shown in FIG. 7 vertebra 20includes a generally cylindrical centrum 300 which bears most of theload of the body and which includes a high percentage of trabecularbone. As noted above, trabecular bone has been determined to be asensitive indicator of bone change in the early stages of osteoporosis.Ideally then, bone density measurements of the spine would primarilymeasure trabecular bone.

Extending in the posterior direction from the centrum 300 are transverseprocesses 302, the inferior and superior articular processes 303, andthe spinal lamina 306. Henceforth, for simplicity, these posteriorstructures will be collectively termed spinal processes 305. The bone ofthe spinal processes 305 is of higher density than the centrum 300 andinclude little trabecular bone.

Referring now to FIG. 8, in an AP bone density image 310, the spinalprocesses 305 (not directly visible) form zones of higher density 312superimposed on the image of the centrum 300. These zones 312, whenaveraged into the vertebral average bone density reading for thevertebra 20 bias the average density upward possibly obscuringclinically significant loss in the trabecular bone mass. For thisreason, it is desirable to identify and eliminate these zones 312 fromthe measurement process.

While it is possible to locate these zones 312 with respect to thelandmarks on the vertebra 20 alone, as projected in the image 310,variations in vertebra 20 make it preferable that these zones bedistinguished by establishing certain threshold levels of bone densityindicative of the zones 312. That is, if the density of a data element31 of the image 310 is above the established threshold, it is assumedthat this data element 31 measures, in significant part, the bone of thespinal processes 305.

The particular density threshold, defining zones 312, will varydepending on the patient. Accordingly, the threshold is determined by areference density measurement made at an established position withrespect to the vertebrae 20. This procedure is performed by the computer18 operating on the matrix 29 of data elements 31 as has previously beendescribed.

Referring momentarily also to FIG. 3, the left and right spinal boundarylines 220 are used to identify the approximate horizontal center of theintervertebral spaces 313. The superior and inferior borders of adjacentvertebra 20, previously detected, are used to determine a verticalcenter of the intervertebral space 313. A vertical and horizontalvertebral center 314 is thus determined.

A cluster of data elements 31 around this center 314 is averaged todetermine a density value of the spinal processes 305 without theintervention of the centrum 300. This value will be used as a referencemeasurement to identify the zones 312.

Each data element 31 within the vertebra 20 is next identified by thepoint typing previously described and the identification of the boundarylines 220 and the intervertebral spaces. Those data elements 31 areanalyzed to find the data element 31 indicating maximum bone density orpeak value within the vertebra 20. The previously determined referencevalue is then subtracted from the peak value to provide a density limitidentifying the zones 312.

Now only data elements 31 within the vertebra 20 having density valuesbeneath this limit are used in the calculation of the vertebral averagebone density for the vertebra 20 thus effectively excluding zones 312from the analysis of vertebral average bone density. Data elements 31having higher values are considered to be upwardly biased by the spinalprocesses 305 and are ignored. The vertebral average bone density isthus the sum of the data elements 31 within the vertebrae 20 excludingzones 312 divided by the area encompassed by those included dataelements 31. This density is an area density, e.g. grams per cm².

Referring again to FIG. 5, the value-based point typing 400 and thetemplate-based point typing 402 are desirably augmented by operatorpoint typing 404 in which the operator interactively changes thecategories of certain pixels 201. This operator point typing enlists thesuperior knowledge of a trained operator in identifying bone and softtissue in the context of a radiographic-like image of the bone and softtissue. The operator point typing also permits use of the system toimage and measure bones for which templates incorporating generalknowledge about the bone anatomy have not been developed. This may occurfor other bones in the body (e.g., hands or feet) or for individualswhose bones do not conform to the generalized rules stored within theequipment. Such flexibility may also be desired if the equipment is tobe used with animal studies.

Important to the operator point typing is the provision of a suitableinterface between the operator and the computer to facilitate theoperator's recharacterization of the particular pixels 201. Such aninterface should, to the extent possible, prevent inadvertent changingof data elements by the operator. The present invention realizes thesegoals by adopting a "paintbrush" interface in which the operatormaneuvers a cursor "paintbrush" over the image to change the pointtyping of selected data elements.

Referring now to FIG. 6, the image 200 is displayed to the operator onthe display 22 together with a menu screen 222 having a brush type menu224 and a brush size menu 226. Such menu systems are well known in thecomputer art and provide graphically for the input of operatorparameters. In particular, the brush type menu 224 offers five differentbrushes: bone, tissue, air, artifact and neutral. When a particularbrush type is selected, the material of the selected brush type ishighlighted in the image 200 with a blue color according to the currentpoint typing. Thus, when bone is selected as the brush type as indicatedin FIG. 6, those pixels 201 previously identified by the point typing ofprocesses 400 and 402 will be highlighted in blue. All materials,including bone, also take a gray scale value based on the values oftheir data elements 31 as has been described. Thus, all the data of thescan is available to the human operator in making determinations ofpoint type.

If the tissue brush type is selected, the tissue pixels 201 will behighlighted in blue and the bone tissues will revert solely to black andwhite gray values. The categories of air and artifact in the presentexample would highlight no tissue as no pixels 201 have beencharacterized as either air or artifacts. The neutral characterizationwill highlight the portions of the ilium 206, the clavicle 208 and theprocesses 302 previously excluded by the value-based and template-basedpoint typing of process box 400 and 402.

Pixels 201 are selected by the operator by use of a "paintbrush" cursor228 whose position may be controlled by the mouse 25 as previouslydescribed with respect to FIG. 1 or other well known cursor controlleddevices. As the mouse 25 is moved, the image of the cursor 228 moves onthe image 200 providing an interactive real time control of point-typingof points by the operator.

After the operator moves the cursor 228 to a particular point on theimage 200, the mouse button 26 may be depressed causing those dataelements corresponding to the region of the image 200 covered by thecursor 228 to be changed to the characterization indicated by the brushtype menu 224. Preferably, the mouse is used dynamically in the mannerof a paintbrush with the button 26 continuously depressed wherein theswept area of the cursor 228 as it is moved over the image 200 in a pathdefines those pixels 201 changed to the new classification.

For example, if the bone brush type is being used, pixels 201 selectedby the operator will be changed into the bone classification.

The brush size may be changed from one sample, that is, one pixel of theimage through square shapes up to 9×9 samples or pixels 201. Thus, forrapid removal of extraneous bone into the neutral classification, alarge paintbrush may be used, whereas a small paintbrush may be used forcorrection of individual point classifications, for example, between theintervertebral spaces.

The paintbrush allows flexible adjustment of the ROI to conform exactlyto the vertebral body.

For measurements of bone density, this operator adjustment ofclassification can significantly enhance the clinical value of themeasurement with minimum risk of affecting its reproducibility. Althoughchanging pixels 201 classified as bone into, for example, neutral hasthe effect of eliminating those pixels 201 from the calculation of bonedensity, it also eliminates those pixels 201 from the divisor used inthe density calculation. Thus, for a homogenous bone recharacterizationof some of its pixels 201 as neutral, for example, will not affect theoverall density measurement. On the other hand, the use of the cursor228 to remove the denser regions of the spinal processes 302 even at theexpense of removing some bone which is substantially trabecular only cansubstantially increase the sensitivity of the density measurement in thedetection of loss of bone mass.

Referring again to FIG. 5, once the point typing is complete the totalbone content for the bone pixels identified to a vertebra 20 may bedetermined per process block 406 and printed out on the display 22.Total bone content is the above computed vertebral average bone density(converted to a per data element 31 value) times the total number ofdata elements 31 within the vertebrae regardless of whether they are inzones 312 or not.

The combination of the intervertebral boundaries and the left and rightboundary lines 220 may be used to accurately define a vertebral regionto be used for calculating bone density for that particular vertebra 20.

In addition, bone density measurements may be made at particular regionsfor entire vertebrae or collections of vertebrae within the spine 202.Prior to these density measurements, the point typing may be used tocalibrate a dual energy algorithm based on a soft tissue reading so asto remove the effects of intervening soft tissue superimposed over thebones of interest.

It is thus envisioned that the present invention is subject to manymodifications which will become apparent to those of ordinary skill inthe art. Accordingly, it is intended that the present invention not belimited to the particular embodiment illustrated herein, but embracesall such modified forms thereof as come within the scope of thefollowing claims.

I claim:
 1. A densitometer comprising:(a) an opposed radiation sourceand detector; (b) a positioner supporting the radiation source anddetector at a predetermined angle about a patient to obtain a twodimensional array of pixels having values representing the attenuationof radiation at locations through the patient corresponding to thelocations of the pixels within the two dimensional array; (c) a digitalcomputer having a display screen and a cursor controller providing aselect signal and cursor coordinates in response to operator commands,the digital computer receiving the array of pixels and operatingaccording to a stored program to:(1) categorize the pixels into at leastthree categories including categories of bone pixels, soft tissue pixelsand neutral pixels; (2) display an image of the pixels in which the atleast three categories are visually distinguishable; (3) respond tocursor coordinates from the cursor controller to move a location of acursor symbol in a path on the image; (4) change the category ofindicated pixels in at least one of the categories for indicated pixelsin the path when the select signal is received; and (5) display a valueto the operator based on the bone pixels and soft tissue pixels butexcluding the neutral pixels, after step (4).
 2. The densitometer ofclaim 1 wherein the stored program distinguishes pixels of at least onecategory in the image by color.
 3. The densitometer of claim 1 whereinthe at least three categories are selected from among the groupconsisting of: air and artifact.
 4. The densitometer of claim 1 whereinthe cursor controller additionally provides a cursor format signal inresponse to operator commands that controls the size of the cursorsymbol with respect to the pixels of the image.
 5. The densitometer ofclaim 1 wherein bone integrity value is bone density and wherein step(5) includes the steps of:(i) summing the values of all the pixels inthe bone pixel category after step (4) to produce a total bone contentvalue; (ii) dividing the total bone content value by the number ofpixels in the bone pixel category after step (4).
 6. The densitometer ofclaim 1 wherein the cursor controller additionally provides a categorysignal in response to operator commands that controls the category intowhich the indicated pixels are changed when the select signal isreceived.
 7. A method of analyzing bone in vivo comprising the stepsof:(a) scanning the body with a beam of radiation to acquire a matrix ofdiscrete data elements each having a value wherein each said dataelement corresponds to a defined location in the body, and wherein thevalue of each data element is related to a physical characteristic ofthe material through which the beam of radiation passes; (b) employing adigital computer to:(1) sort the data elements based on their valuesinto bone data categories indicating the physical characteristic of thevertebrae; (2) resort the sorted data elements into the bone datacategories indicating the physical characteristic of the vertebrae basedon their values determined in step (1) and based on their definedlocations; (3) recategorize the data elements into the bone datacategories indicating the physical characteristic of the vertebrae basedon operator review of a display of the resorting of step (2) and adisplay of the data elements indicating their values and definedlocations.
 8. The method of claim 7 wherein the value of each dataelement is displayed in step (3) in the form of a brightness of adisplayed point at a displayed location corresponding to the definedlocation of the data element and wherein the resorting of step (2) ofthe data element is indicated by a color of the displayed point.